The present invention relates to a rotation control device for a sewing motor.
Heretofore, a sewing motor is controlled so as to be rotate and brake in accordance with a predetermined sequence known as invertor drive. In such a rotation control device, if the output torque of the sewing motor is excessively large, an undue load is imposed on a mechanical system at the time of acceleration or deceleration of the sewing motor. Hence the mechanical system is liable to be damaged. To prevent the sewing mechanism from being imposed on an excessive load, the current flowing in each winding of the motor is set to be less than a maximum allowable level.
However, the limitation of the motor torque weakens the penetrating force of a needle when stitching a thick fabric. An industrial sewing machine, in particular, is often required to sew a very thick work having a plurality of superimposed thick fabrics. In some cases, the D.C. motor is nearly locked in sewing such a very thick work due to the high resistance of the very thick work to the penetrating action of a needle.
In the sewing machine of the type having an arrangement wherein the driving force generated from the sewing motor is transmitted to the mechanical system through a transmission belt and pulleys, the torque to be transmitted to the mechanical system can be increased if the diameter of the motor pulley is reduced. However, if this is done, the maximum driving speed which may otherwise be achieved if the diameter of the motor pulley is remained unchanged is adversely lowered.